Installation guide

The quorum disk daemon (qdiskd) runs on each node in the cluster, periodically evaluating its

own health and then placing its state information into an assigned portion of the shared disk

area. Each qdiskd then looks at the state of the other nodes in the cluster as posted in their

area of the QDisk partition. When in a healthy state, the quorum of the cluster adds the vote

count for each node plus the vote count of the qdisk partition. In the above example, the total

vote count is five; one for each node and two for the qdisk partition.

If, on any node, qdisk is unable to access its shared disk area after several attempts, then the

qdiskd on another node in the cluster will attempt to fence the troubled node to return it to an

operational state.

Additional heuristics

Red Hat adds an additional feature to the quorum disk mechanism. Optionally, one or more

heuristics can be added to the qdisk configuration. Heuristics are tests performed by the qdisk

daemon to verify the health of the node on which it runs. Typical examples are verifications of

network connectivity such as the server's ability to ping network routers. Heuristics can also

be used to implement network tiebreaker functionality.

4.5 GFS

Red Hat's Global File System (GFS) is a POSIX compliant, symmetric shared cluster file

system. GFS lets servers share files with a common file system on a SAN.

With local file system configurations such as ext3, only one server can have access to a disk

or logical volume at any given time. In a cluster configuration, this approach has two major

drawbacks. First, active/active file system configurations cannot be realized, limiting scale out

ability. Second, during a failover operation, a local file system must be be unmounted from the

server that originally owned the service and must be remounted on the new server.

GFS creates a common file system across multiple SAN disks or volumes and makes this file

system available to multiple servers in a cluster. Scale out file system configurations can be

easily achieved. During a failover operation, it is not necessary to unmount the GFS file

system because data integrity is protected by coordinating access to files so that reads and

writes are consistent across servers. Therefore, availability is improved by making the file

system accessible to all servers in the cluster. GFS can be used to increase performance,

reduce management complexity, and reduce costs with consolidated storage resources.